Enthalpy of Formation of NH

Abstract

Measurements have been made of [NH], [NH₃], and [OH] as a function of distance from the reaction zone of a flat NH₃/O₂/N₂ flame at atmospheric pressure. The function [NH] [H₂O]²/[NH₃] [OH]² is shown to behave like the equilibrium constant for the reaction ${\mathrm{NH}}_3\text{+2}\mathrm{OH}\to \mathrm{NH}\text{+2}{\mathrm{H}}_2\mathrm{O}$ , but to be about a factor of 10 smaller than the value calculated from the JANAF tables. In order to account for this discrepancy, various uncertainties in experiment and in input data are analyzed. Of these, special attention is given to the extinction coefficient of NH₃ near 2300 Å and to the possibility that [NH] is depressed below its equilibrated value by kinetic effects. These uncertainties are found to be not large enough to account for the observations. It is concluded that the discrepancy can best be resolved by adopting an enthalpy of formation of NH of $\text{90 ± 4\hspace{0.17em}}\mathrm{kcal}/\mathrm{mole}$ , rather than 81 kcal/mole as in the JANAF tables.